The present invention is concerned with a piston for an engine or motor, e.g. for an internal combustion engine.
A piston for an internal combustion engine normally comprises a crown and a skirt. The crown may be made of metal, such as aluminium, an alloy of aluminium, or a ferrous material, or may be made of a ceramic material. The crown provides a surface against which the force of the combustion gases acts and provides grooves in which rings are mounted to seal the piston against the surface of a bore in which the piston reciprocates. The skirt of such a piston serves to guide the piston in said bore.
In many cases, the crown and skirt of a piston are of integral construction of the same material. Normally, the skirt provides two spaced bores which receive a gudgeon pin by which the piston is connected to a connecting rod. It is, however, known (see GB Patent Specification No. 1,422,776) to provide a gudgeon pin boss which depends from the crown and provides the bore for receiving the gudgeon pin, the metal piston skirt encircling the boss. This arrangement allows a shorter and, therefore, lighter gudgeon pin to be used. Such a boss may be in two portions to support opposite ends of the gudgeon pin.
It is known for the skirt to be made as a different piece from the crown and to be secured thereto, this being done in the interests of ease of manufacture. The different piece may also be of a different material from the crown or to be coated with a different material. Where the skirt is a different piece, it may be joined to the crown through the gudgeon pin (see GB 2188122A) or may be supported partially by the crown and partially by the gudgeon pin boss (see GB 2129523A) so that a rigid skirt is formed. The skirt may be of plastics material or may be coated with plastics material.
As such pistons reciprocate in the operation of an engine or motor, they are subject to lateral forces tending to press them against the wall of the bore in which they reciprocate. This creates noise, known as "slap noise", which becomes more significant with modern quieter engines. The use of a rigid plastics or plastics coated skirt reduces slap noise because of the inherent sound-damping characteristics of plastics materials, but the slap noise is still significant.
It is an object of the present invention to provide a piston in which slap noise is reduced.